To determine if magnesium sulfate has an effect on the development of cystic periventricular leukomalacia in preterm infants, this retrospective case control study was conducted. There were 23,382 infants born at three teaching hospitals in the metropolitan New York area from January 1992 to December 1994. Four hundred ninety-two infants met our entrance criteria. Criteria included a birth weight less than 750 g, survival to at least 7 days of life and at least one cranial ultrasound after 7 days of life. Infants exposed to magnesium sulfate in utero were less likely to develop periventricular leukomalacia. Two of 18 (11%) infants with periventricular leukomalacia were exposed to magnesium sulfate in-utero compared to 14 of 36 controls (39%) (p = 0.035) (OR = 0.196, 95% Cl = 0.039-0.988). Pre-eclampsia as an independent factor was not associated with a reduced risk (p = 0.251) (OR = 0.294, 95% Cl = 0.033-2.65). Preterm infants exposed to antenatal magnesium sulfate were found to have a reduced risk of developing cystic periventricular leukomalacia.